1. Field of the Invention
The present invention relates generally to the manufacture of bi-metallic tubes or pipes and, more particularly, to a bi-metallic extrusion billet preform used in the production of such tubes or pipes, and a method and apparatus for providing such preforms.
2. Description of the Related Art
The operating conditions in many industrial processes require the use of corrosion resistant components. Such corrosion resistant components include tubes or pipes which are directly exposed to the combustion process or to the material involved in the chemical process. Well known examples of such processes include the high corrosion areas of fossil-fueled steam generators firing high chlorine coals, steam generators for waste Kraft liquor, or other types of chemical processing equipment.
A combination of suitable corrosion resistance and mechanical properties is often required, and in many situations conditions on the "water" side and "gas" side of the tubes in the steam generator require different alloy chemistries. A prior art solution to the problem of producing a tubular component suitable for exposure to two different environments, one on the inside of the tube and the other on the outside thereof, is the bi-metallic tube.
One prior art method of producing such bi-metallic tubes is the hot coextrusion process, which is designed to produce a metallurgical bond between inner and outer layers of the tube. Coextruded tubes with stainless steel type 304 and 310 claddings and carbon or low alloy steel substrates have been produced and used widely in the aforementioned applications. The steps of such a typical prior art coextrusion process to produce such a tube comprise:
1. Sleeves of the two alloys are machined to close tolerances and fitted together to form a composite billet. PA0 2. The ends of the sleeve are welded together to prevent ingress of air during preheat and extrusion. PA0 3. The welded billets are preheated and coextruded using standard extrusion practices for stainless tubing, including the use of glass lubricants. PA0 4. When the tube is in its hot finished condition, the glass lubricant is removed and the tube is heat treated to obtain any required mechanical properties. PA0 5. To further reduce the tube diameter, cold rolling or pilgering may be used followed by appropriate heat treatments. PA0 6. The finished tube is extensively tested, especially to verify bond integrity between the layers.
Bi-metallic tubes produced by the aforementioned hot coextrusion process have performed satisfactorily; their major drawback is their relatively high cost. Generally, a low-alloy steel tube with a 2-3 mm cladding of, for example, type 310 stainless steel, costs 7-9 times as much as a low-alloy steel tube, and as much or more than a monolithic tube made of the cladding alloy. Again, certain requirements such as operating conditions and various mandatory boiler codes and the like may prohibit the use of a corrosion resistant monolithic tube made of certain materials in a given environment. Reasons given for such high costs include the cost of billet preparation and the relatively high yield losses due to the large discards at both ends of the finished tube.
Accordingly, since one of the reasons for the high cost of producing a bi-metallic tube by the hot coextrusion process is the cost of producing the initial bullet, it has become desirable to develop a new bi-metallic extrusion billet preform that can be utilized in the prior art hot coextrusion processes but which can be produced at a much lower cost than in the prior art method.